Aliphatic amines are important organic intermediates which are prepared on a large industrial scale. They are further processed for the preparation of agrochemicals or dyes for example, or they are used as additive in surface-active formulations, as a corrosion inhibitor in lubricants or as auxiliaries in the paper, textile and rubber industries.
The preparation of primary aliphatic amines from aldehydes and ammonia with hydrogen over a catalyst is known. This reaction is also referred to as reductive amination. The amine formation can be described by the following reaction stages:R—C(═O)H+NH3→R—C(═NH)H+H2O  (1)R—C(═NH)H+H2→R—CH2—NH2  (2)
The first reaction stage initially eliminates water to produce an imine, which is then hydrogenated catalytically in a second reaction stage.
However, unwanted secondary reactions occur as the reaction is conducted. Firstly, the feed aldehydes may be directly hydrogenated to the alcohol. Secondly, the feed aldehyde may undergo an aldol condensation in the basic medium and primary amine which has already been formed can react with the feed aldehyde via the azomethine intermediate to form a secondary amine which can then further react in a similar fashion to form a tertiary amine. Moreover, the aldol condensation products contain reactive groups which together with the nitrogen-containing compounds can form comparatively high-boiling condensation products. To improve selectivity in the direction of the primary aliphatic amines and to suppress the formation of high-boiling by-products, various measures have been proposed in the prior art, for example using excess ammonia or a solvent when it is likely that the reaction mixture will become inhomogeneous as a result of the water formed (Houben-Weyl, Methoden der organischen Chemie, 4th edition, Georg Thieme Verlag Stuttgart, Volume XI/1, p. 602 ff.).
DE 936211 describes a liquid-phase process for preparing primary aliphatic amines. In this process, the aldehyde to be reacted is first mixed with ammonia at temperatures below 0° C. Optionally, the aldehydes are diluted with a low-boiling alcohol, for example methanol. This mixture is then catalytically hydrogenated, for example over a cobalt or nickel catalyst, at elevated temperature and pressure in the liquid-phase mode or trickle mode.
According to DE 199 35 448 A1, a mixture of methanol and ammonia is admixed with Raney nickel and, following pressurization with hydrogen, is heated to reaction temperature. The aldehyde is then added. After completion of the reaction, the batch is depressurized and methanol and ammonia evaporate. The primary aliphatic amine which remains is then further reacted.
DE 10 2010 045 142 A1 discloses a solvent-free process in the liquid phase in which the feed aldehyde is reacted with hydrogen and ammonia in the presence of a hydrogenation catalyst at a temperature of 100 to 170° C. and a pressure of 6 to 11 MPa and wherein at least 30 mol of ammonia are used per mole of feed aldehyde.
The prior art also refers to using ammonia under supercritical conditions in the reductive amination process. The critical temperature of ammonia is 132.5° C. and the critical pressure is 112.5 atm (Handbook of Chemistry and Physics, 50th Edition 1969, The Chemical Rubben CO, p. F-64), corresponding to 11.4 MPa. According to GB 1,421,278, the reaction temperature is limited to 160° C. and the superatmospheric pressure up to 125 atm (corresponding to 12.7 MPa). In a preferred embodiment, the molar ratio of ammonia to feed aldehyde is restricted to 16 to 1. Working example 4 in GB 1,421,278 discloses the reductive amination of isobutyraldehyde with ammonia at a reaction temperature of 140° C. and a superatmospheric pressure of 125 atm (12.7 MPa superatmospheric pressure).
According to WO97/38955 A1, the reductive amination of aldehydes is carried out in a continuous process in the presence of a heterogeneous catalyst, wherein at least one reaction partner, in addition to hydrogen, is in the supercritical state or close to the supercritical state. The reaction is preferably conducted in the presence of a solvent, which is itself present in the supercritical state under the reaction conditions, such that hydrogen and the other reaction components are present in a homogeneous phase. It cannot generally be deduced from the prior art that aliphatic aldehydes having a mid-range carbon number are converted with high selectivity to primary aliphatic amines under conditions above the critical conditions for ammonia. The formation of high-boiling by-products is frequently observed which decreases the selectivity and thereby the yield of the desired primary aliphatic amine. In addition, the methods according to the prior art are prone to only a limited catalyst service life, whereby, besides the decreased yield of the desired amine mentioned, economic disadvantages are also associated with the amination process.
It is therefore an object of the present invention to provide a process for preparing primary aliphatic amines which is technically simple and in which the desired primary aliphatic amines are obtained with high selectivity. In particular, the formation of high-boiling by-products should be suppressed as far as possible. At the same time the service life of the hydrogenation catalyst should also be extended.